Discrete bypass particle concentrator

ABSTRACT

A discrete bypass particle concentrator can significantly reduce surface fouling and manufacturing cost by including bypass stages after each of concentration stages.

PRIORITY CLAIM

This application claims priority to U.S. Provisional Patent Application No. 61/969,038, filed Mar. 21, 2014, which is incorporated by reference in its entirety.

GOVERNMENT SPONSORSHIP

This invention was made with government support under Contract No. FA8721-05-C-0002 awarded by the U.S. Air Force. The government has certain rights in the invention.

BACKGROUND

Cohesive, narrow beams of micron-scale particles—coalesced from atmospheric aerosols or particulate feedstocks—are necessary in numerous applications, among them spectroscopy and highly-precise material deposition processes. There are few low-power, high-throughput particle concentration methods that are able to yield a continuous beam of focused particles. Overall system power and space budgets further restrict available design options. Focusing via aerodynamic lenses (ADLs) are one way to satisfy these various goals and constraints.

SUMMARY

This invention provides a means of concentrating micron-scale particles, suspended in the ambient air or from a particulate feedstock, into a highly focused particle beam. A device for concentrating particles in a flow can include a housing including a plurality of stages wherein each stage includes an inlet, an outlet, and a flow channel, wherein the flow channel includes an inside diameter and connects the inlet and the outlet, and the inside diameter of the flow channel is decreasing each subsequent stage, and a bypass between the outlet of a stage and the inlet of the following stage, wherein the flow that exists from a stage divides into a bypass flow that exits through the bypass and a particle flow that enters into the inlet of the flowing stage, wherein the main flow that exits from the last stage exits the device as a particle beam, wherein the bypass flow include less particles than the main flow.

The plurality of stages can be in a series with the inside diameter of the flow channel decreasing in each subsequent stages. The opening of the bypass can be smaller than the opening of the inlet of the following stage. The bypass flow can form a bypass annular sheath flow, and the particle beam and the bypass annular sheath flow can be collimated.

The size of flow channel and flow rate can be configured for operation at Reynolds number of less than 2300, or more preferably less than 1200. Flow rates can range from 0.5 L/min to 30 L/min.

The length of the device can be less than 20 cm. Particle sizes can be between 0.25 μm and 25 μm, or between 1 μm and 10 μm.

Other aspects, embodiments, and features will be apparent from the following description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a discrete bypass particle concentrator.

FIG. 2 is a diagram of aerodynamic lenses.

FIG. 3 is a diagram of Slit-type particle concentrator with minor/major flow split.

FIG. 4 is a diagram of discrete bypass particle concentrator.

FIG. 5 is a diagram of CAD model of discrete bypass particle concentrator designed for 40× concentration of 1-10 micron particles in 10 L/min flow.

FIG. 6 is a longitudinal cross-sectional view of FIG. 4

FIG. 7 is a graph depicting predicted concentration performance of demonstration discrete bypass particle concentrator

FIG. 8 is a simulation of velocity magnitude of 10 L/min flow through discrete bypass particle concentrator

DETAILED DESCRIPTION

The discrete bypass particle concentrator provides a means of concentrating micron-scale particles, suspended in the ambient air or from a particulate feedstock, into a highly focused particle beam. It accomplishes this using a series of concentrating lens stages in parallel with bypass stages, reducing the overall flow Reynolds number of the particle beam. At the final stage, the diverted bypass flow forms an annular sheath flow around the particle beam, minimizing beam expansion.

FIG. 1 shows a general structure of a discrete bypass particle concentrator. A discrete by pass particle concentrator can include a housing 1 including an inlet 2 an outlet 3, and a flow channel 4, wherein the flow channel connects the inlet and the outlet, and the inside diameter of the flow channel is decreasing each stage 5, and a bypass between the outlet 6 of a stage and the inlet 7 of the following stage, wherein the flow that exists from a stage divides into a bypass flow 8 that exits through the bypass and a particle flow 9 that enters into the inlet of the flowing stage, wherein the main flow that exits from the last stage exits the device as a particle beam 10.

Aerodynamic lenses are one of the few high-throughput devices capable of generating a focused, continuous stream of particles. They are made of a series of converging-diverging nozzles, or lenses, as shown in FIG. 2. See, X. Wang and P. H. McMurry, “A Design Tool for Aerodynamic Lens Systems,” Aerosol Science and Technology, p. 320-334, 2006, which is incorporated by reference in its entirety.

These lenses, often consisting of a simple orifice plate, separate the particles from the carrier gas via acceleration and deceleration of the flow. The Stokes number, a ratio of characteristic flow and particle relaxation times, indicates whether or not the particles' inertia and carrier gas flow rate will result in particle focusing along the centerline after passing through the lens. Concentration of a range of particle sizes is possible by using a series of lenses, with smaller particles concentrated as each downstream stage. Concentration performance is heavily dependent on maintaining a laminar flow.

While a Reynolds number of 2300 is often cited as the point of transition to turbulent flow, most concentrators set 1200 as a design limit. The need to maintain a laminar flow limits the final stage lens diameter and by extension the smallest particle size that may be concentrated. One alternative would be to decrease the flow rate or use a less dense carrier gas, though neither of these options are viable for most applications.

A solution to this problem is to split the flow before the last concentration stage (FIG. 3). See J. Goo, Journal of Aerosol Science, p. 1493-1507, 2002, which is incorporated by reference in its entirety. The major “clean” flow, which makes up the bulk of the flow, is skimmed off and the minor particle-laden flow, located on the centerline, continues through the final lens.

This effectively reduces the flow rate through the last concentration stage, thereby maintaining a laminar flow. However, the large pressure gradient in the region of the minor/major flow split may disrupt the particle beam and contribute to surface fouling, reducing performance over time. Physical tolerances in this area are also extremely tight, significantly increasing manufacturing costs. Explicit segregation of minor/major flows requires an additional sheath flow path downstream to prevent beam dispersion.

The discrete bypass concentrator introduces additional flow bypass stages after each of the concentrating lenses. A diagram illustrating this concept is shown in FIG. 4. A discrete bypass concentrator can include a plurality of stages (i.e. lenses) where each stage includes an inlet, an outlet, and a flow channel, wherein the flow channel connects the inlet and the outlet, and the inside diameter of the flow channel is decreasing each stage, and a bypass between the outlet of a stage and the inlet of the following stage, wherein the flow that exists from a stage divides into a bypass flow that exits through the bypass and a particle flow that enters into the inlet of the flowing stage, wherein the main flow that exits from the last stage exits the device as a particle beam. The concentration stages can be positions in a series so that the main particle flow path would be straight to the exit. The size of the opening to a bypass flow is smaller than the size of the inlet flow, and the majority of the particles stay within the main flow. By splitting the flow at each stage, the ratio of particles to carrier gas at each concentration stage increases while the associated flow rate decreases. This permits more aggressive sizing of lenses and a possible reduction in the number of concentration stages. After the last concentration stage, the bypass flow from each stage forms a stabilizing, annular sheath flow around the tightly collimated particle beam, without the need for additional flow components. Distribution of the bypass flow also reduces the manufacturing tolerance requirements, yielding reduced fabrication costs.

The length of the device can vary. For example, it can be less than 1 m, less than 50 cm, less than 30 cm, or less than 15 cm. The diameter of the device can be less than 10 cm, less than 5 cm, less than 3 cm, or less than 1.5 cm. The length of each concentration stage can be less than 10 cm, less than 5 cm, less than 3 cm, or less than 1 cm. The size of the opening of the exit from the last stage can be less than 1 cm, less than 0.5 cm, less than 0.3 cm, or less than 0.15 cm.

The number of stages in the device can vary. For example, the device can have 2, 3, or more than 4 stages.

The size of the gap between the stages for the bypass flow can be between 0.1 mm and 1 cm. It can be less than 1 mm, less than 3 mm, less than 5 mm, or less than 1 cm. The flow rate can be between 0.5 L/min and 30 L/min. It can be less than 1 L/min, less than 5 L/min, less than less than 10 L/min, less than 20 L/min, or less than 30 L/min. The particle size can be between 0.25 μm and 25 μm. It can be less than 1 μm, less than 3 μm, less than 5 μm, less than 10 μm, less than 15 μm, or less than 25 μm.

An implementation of this approach has been designed to provide at least 40× concentration of 1-10 micron particles in a 10 L/min flow (FIGS. 5-6).

The resulting three-stage discrete bypass particle concentrator was simulated using computational fluid dynamics (CFD) and demonstrated a predicted 100× concentration of 4-10 micron particles, and high concentration factors for 1-3 micron particles (FIG. 7).

This performance is attributed to the flow balance between the particle and bypass flows (FIG. 8). Initial simulations indicate that the pressure drop generated by this concentrator is at least one order or magnitude lower than for comparable ADLs, resulting in decreased power requirements. Adjustments in lens shape and size, bypass gap length, and number of stages will allow for the successful application of this low-power approach to a wide range of flow rates and particle sizes.

Other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A device for concentrating particles in a flow comprising: a housing including a plurality of stages wherein each stage includes an inlet, an outlet, and a flow channel, wherein the flow channel includes an inside diameter and connects the inlet and the outlet, and the inside diameter of the flow channel is decreasing each subsequent stage; and a bypass between the outlet of a stage and the inlet of the following stage, wherein the flow that exists from a stage divides into a bypass flow that exits through the bypass and a particle flow that enters into the inlet of the flowing stage, wherein the main flow that exits from the last stage exits the device as a particle beam. wherein the bypass flow include less particles than the main flow.
 2. The device of claim 1, wherein the plurality of stages is in a series with the inside diameter of the flow channel decreasing in each subsequent stages.
 3. The device of claim 1, wherein the opening of the bypass is smaller than the opening of the inlet of the following stage.
 4. The device of claim 1, wherein the bypass flow forms a bypass annular sheath flow.
 5. The device of claim 4, wherein the particle beam and the bypass annular sheath flow are collimated.
 6. The device of claim 1, wherein the size of flow channel and flow rate are configured for operation at Reynolds number of less than
 2300. 7. The device of claim 1, wherein the size of flow channel and flow rate are configured for operation at Reynolds number of less than
 1200. 8. The device of claim 1, wherein the length of the device is less than 20 cm.
 9. The device of claim 1, wherein the size of a particle is between 1 μm and 10 μm. 